The following disclosure is based on German Patent Application No. 10127227.8 filed on May 22, 2001, which is incorporated into this application by reference.
1. Field of Invention
The invention relates to a catadioptric projection lens for imaging a pattern arranged in an object plane onto an image plane.
2. Description of the Related Art
Projection lenses of said type are employed on projection illumination systems, in particular wafer scanners or wafer steppers, used for fabricating semiconductor devices and other types of micro-devices and serve to project patterns on photomasks or reticles, hereinafter referred to generically as xe2x80x9cmasksxe2x80x9d or xe2x80x9creticles,xe2x80x9d onto an object having a photosensitive coating with ultrahigh-resolution on a reduced scale.
In order to create even finer structures, it will be necessary to both increase the numerical aperture (NA) of the projection lens to be involved on its image side and to employ shorter wavelengths, preferably ultraviolet light with wavelengths less than about 260 nm.
However, there are very few materials, in particular, synthetic quartz glass and crystalline fluorides, such as calcium fluoride, magnesium fluoride, barium fluoride, lithium fluoride, lithium calcium aluminum fluoride, lithium strontium aluminum fluoride, and similar, that are sufficiently transparent in that wavelength region available for fabricating the optical elements required. Since the Abbxc3xa9 numbers of those materials that are available lie rather close to one another, it is difficult to provide purely refractive systems that have been sufficiently well-corrected for chromatic aberrations. Although this problem could be solved by employing purely reflective systems, fabricating such mirror systems requires substantial expense and effort.
In view of the aforementioned problems, catadioptric systems that combine refracting and reflecting elements, i.e., in particular, lenses and mirrors, are primarily employed for configuring high-resolution projection lenses of the aforementioned type.
Whenever imaging reflective surfaces are employed, it will be necessary to use beam-deflecting devices if images free of obscurations and vignetting are to be achieved. Both systems having one or more deflecting mirrors and systems having solid beam-splitters are known. Additional plane mirrors may also be employed for folding the optical path. Folding mirrors are usually employed only in order to allow meeting space requirements, in particular, in order to orient the object and image planes parallel to one another. However, folding mirrors are not absolutely necessary from the optical-design standpoint.
Employing systems having a solid beamsplitter in the form of, e.g., a beamsplitter cube (BSC), has the advantage that it allows implementing on-axis systems. Polarization-selective reflective surfaces that either reflect or transmit incident radiation, depending upon its predominant polarization direction, are employed in such cases. The disadvantage of employing such systems is that hardly any suitable transparent materials are available in the desired, large volumes. Moreover, fabricating optically active beamsplitter coatings situated within beamsplitter cubes is extremely difficult. Heating effects occurring within beamsplitters may also present problems at high radiant intensities, since inside the beamsplitters an intermediate image is created.
One example of such a system is depicted in European Pat. No. EP-A-0 475 020, which corresponds to U.S. Pat. No. 5,052,763, where the mask involved lies directly on a beamsplitter cube and the intermediate image formed lies within the beam-splitter cube, behind its internal beamsplitting surface. Another example is depicted in U.S. Pat. No. 5,808, 805 and the associated application for continuation of same, U.S. Pat. No. 5,999,333, where a multi-element compound-lens group with a positive refractive power lies between the object plane and a beamsplitter cube. The collected light beam is initially deflected toward a concave mirror by the beamsplitter cube and then reflected back to the beamsplitter cube and through its beamsplitting surface toward the aforementioned compound-lens group with a positive refractive power by the concave mirror. The intermediate image lies within the beamsplitter cube, in the immediate vicinity of its beamsplitting surface. However, none of these documents makes any statements regarding heating problems that might arise or how they may be avoided.
European Patent No. EP-A-0 887 708 states measures for avoiding thermally induced imaging errors for a catadioptric system having a beamsplitter cube, but apparently no intermediate image falling within its beamsplitter cube. The intention here was obtaining a symmetric distribution of radiant intensity over the beam-splitter cube""s beamsplitting surface, i.e., a distribution that would yield a heating profile symmetrically distributed over the beam-splitter""s beamsplitting surface, by suitably routing the beam transiting the beamsplitter cube. It was stated that the resultant wave-front distortions, such as those that result from nonuniform heating, which are difficult to eliminate, were avoidable.
Some of these disadvantages of systems having beamsplitter cubes may be avoided in the case of systems having one or more deflecting mirrors in their beam-deflecting device. However, such systems have the disadvantage that they are, by virtue of their design, necessarily off-axis systems.
A catadioptric reduction lens of that type is described in European Pat. No. EP-A-0 989 434, which corresponds to U.S. Ser. No. 09/364382. These types of lenses have a catadioptric first section having a concave mirror and a beam-deflection device that is followed by a dioptric second section arranged between their object plane and their image plane. Their beam-deflecting device, which is configured in the form of a reflecting prism, has a first reflective surface for deflecting radiation coming from their object plane to a concave mirror and a second reflective surface for deflecting radiation reflected by that concave mirror to a second section containing exclusively refractive elements. Their catadioptric first section creates a real intermediate image that lies slightly behind this prism""s second reflective surface and well ahead of the first lens of their second section. Their intermediate image is thus readily accessible, which may be taken advantage of for, e.g., installing a field stop.
Another reduction lens that has a beam-deflection device having a deflecting mirror is described in U.S. Pat. No. 5,969,882, which corresponds to European Pat. No. EP-A-0 869 383. This system""s deflecting mirror is arranged such that light coming from its object plane initially strikes the concave mirror of its first section, where it is reflected to the system""s beam-deflecting device""s deflecting mirror, where it is reflected to a second reflective surface, where it is deflected toward the lens of the system""s exclusively dioptric second section. The elements of this system""s first section that are utilized for creating its intermediate image are configured such that its intermediate image lies close to its beam-deflecting device""s deflecting mirror. Its second section refocuses its intermediate image onto its image plane, which may be oriented parallel to its object plane, thanks to the reflecting surface that follows its intermediate image in the optical train.
U.S. Pat. No. 6,157,498 depicts a similar configuration whose intermediate image lies on, or near, the reflective surface of its beam-deflecting device. Several lenses of its second section are arranged between its beam-deflecting device and a deflecting mirror located in its second section. In addition, an aspheric surface is arranged in the immediate vicinity of, or near to, its intermediate image exclusively for the purpose of correcting for distortions, without affecting other imaging errors.
A projection lens having a reducing catadioptric section and an intermediate image in the vicinity of the deflecting mirror of a beam-deflection device is depicted in German Pat. No. DE 197 26 058.
The U.S. patent mentioned above, U.S. Pat. No. 5,999,333, depicts another catadioptric reduction lens having deflecting mirrors for which light coming from its object plane initially strikes a concave mirror, where it is reflected to the lens"" beam-deflecting device""s sole reflective surface. The intermediate image created by its catadioptric section lies close to this reflective surface, which reflects light coming from that concave mirror to a dioptric second section that images this intermediate image onto its image plane. Both its catadioptric section and its dioptric section create reduced images.
A similarly configured lens for which the intermediate image created by its catadioptric section lies near its deflecting device""s sole reflective surface is depicted in Japanese Pat. No. JP-A-10010429. The surface of the lens of the following dioptric section that lies closest to the deflecting mirror is aspheric in order that it may make a particularly effective contribution to correcting for distortions.
Those systems whose intermediate image lies near, or on, a reflective surface may be compactly designed. They also allow keeping the field curvatures of these systems, which are off-axis illuminated, that will need to be corrected small. One of their disadvantages is that even the slightest flaws on any of their reflective surfaces may adversely affect the qualities of images projected onto their image plane. Moreover, their focusing of radiant energy onto reflective surfaces may cause heating effects that might adversely affect their imaging performance. The resultant, locally high, radiant intensities may also damage the reflective coatings that are normally applied to the surfaces of mirror blanks.
The problem addressed by the invention is avoiding the disadvantages of the state of the art. One particular object is to provide a projection lens whose imaging performance will be relatively insensitive to fabrication tolerances.
As a solution to these and other objects, the invention, according to one formulation, provides a catadioptric projection lens for imaging a pattern situated in an object plane of the projection lens onto an image plane of the projection lens while creating a real intermediate image, which includes:
a catadioptric first section with a concave mirror and a beam-deflecting device located between said object plane and the image plane; and
a dioptric second section arranged following the beam-deflecting device;
wherein the second section starts after a final reflective surface of the catadioptric section and includes at least one lens arranged between the final reflective surface and the intermediate image.
Beneficial embodiments thereon are stated in the dependent claims. The wording appearing in all of the claims is herewith made a part of the contents of this description.
A projection lens in the sense of the invention that is of the type mentioned at the outset hereof is characterised in that its second, dioptric section, which starts behind the final reflective surface of its beam-deflecting device, has at least one lens arranged between said final reflective surface and its intermediate image. Said intermediate image thus lies within its second, exclusively refractive, section in order that at least one of the lenses of said second section that precede said intermediate image in the optical train may contribute to creating said intermediate image. The invention thus foresees that the distance between said final reflective surface of said beam-deflecting device and said intermediate image will be considerable, which may allow, e.g., creating an accessible intermediate image in order to, e.g., allow installing a field stop in order to reduce stray-light levels. It will be particularly beneficial if that large distance will provide that said final reflective surface lies in a zone where the beam diameter is rather large, which will provide for its uniform illumination while avoiding hazardous, localized, peaks in radiant intensity and spread any heating of the optical element to which said reflective surface has been applied over a larger area, which will, in turn, improve its imaging performance. More important, however, is that any minor flaws that may be present on its reflective surface will have only a negligible, or no, effect on the qualities of images projected onto the image plane. Lenses with high imaging performance may thus be constructed, in spite of the minimal demands on the uniformity and figure of said final reflective surface.
The term xe2x80x9cfinal reflective surface,xe2x80x9d as used here, is to be interpreted as referring to that reflective surface that lies immediately ahead of said intermediate image in the optical train, where said surface may be a polarization-selective beamsplitting surface of a beamsplitter cube (BSC) or the surface of a highly reflective deflecting mirror, which may be preceded by another deflecting mirror of a beam-deflecting device in the optical train. Rear-surface mirrors in the form of deflecting prisms are also feasible. In the case of projection lenses according to the invention, said xe2x80x9cfinal reflective surfacexe2x80x9d concludes their catadioptric section. Said final reflective surface may be followed by another reflective surface that causes a beneficial, from the structural standpoint, folding of said projection lens"" optical path that has been added at the entrance to, or between the lenses of, said section in order to, e.g., allow orienting said projection lens"" object and image planes parallel to one another.
Said optical element between said final reflective surface and said intermediate image that has been termed a xe2x80x9clensxe2x80x9d here may also differ from conventional lenses in form and function and may be in the form of, e.g., a planar plate having an aspheric correction, a truncated lens, or a half-lens. The term xe2x80x9clens,xe2x80x9d as used here, thus, in general, refers to any transparent optical medium that optically affects transmitted radiation.
The aforementioned benefits apply regardless of whether a lens is arranged between said final reflective surface and said real intermediate image, largely due to the large distance between same. Said distance, which shall hereinafter also be referred to as the xe2x80x9cintermediate-image distance,xe2x80x9d should preferably be chosen such that the diameter of the beam at a surface orthogonal to said optical axis at the intersection of said final reflective surface with said optical axis will be at least 10% of the diameter of said concave mirror, e.g., 17% or more of said diameter. However, said distance should not be so large that said ratio of the diameter of said beam to the diameter of said concave mirror will be much greater than 20% or 25% in order to confine the field curvatures that will need to be corrected to manageable levels. Said large intermediate-image distance will allow arranging said at least one lens between said final reflective surface and said real intermediate image, where said lens or lenses will preferably have a positive refractive power or powers, which will keep the diameter of those lenses that follow said intermediate image small, which, in turn, will allow designing said second section in manners that will allow reducing the quantities of materials required.
Arranging at least one lens between said final reflective surface and said real intermediate image also provides hitherto unknown opportunities for minimizing, or totally eliminating, the deleterious effects of lens heating. In order to reduce or preclude same, a preferred embodiment of the invention has a front intermediate-image lens arranged on its object side, ahead of said intermediate image, and a rear intermediate-image lens arranged on its image side, behind said intermediate image, where said intermediate-image lenses are symmetrically arranged with respect to said intermediate image such that any asymmetric contributions to imaging errors, such as coma, caused by heating of said intermediate-image lenses will be partially compensated, even nearly fully compensated, as shall be discussed in greater detail in terms of the sample embodiments to be discussed below.
The aforementioned symmetric arrangement of said front and rear intermediate-image lenses employed for partially or fully compensating for the effects of asymmetric heating of lenses situated in the vicinity of said intermediate image will be beneficial for both projection lenses of said type and other optical imaging systems that create at least one real intermediate image.
Obtaining the favorable arrangement of said intermediate image according to the invention will be simplified if said first, catadioptric section does not contribute, or does not materially contribute, to the overall reduction ratio of said projection lens. Said catadioptric first section of said projection lens should preferably have a magnifications, xcex2M, that exceed 0.95 and preferred embodiments of same will have magnifications of xcex2M greater than 1, i.e., will create enlarged intermediate images, which will facilitate shifting same to said refractive second section.
In order to keep the field curvatures that will need to be corrected small in spite of said favorable arrangement of said intermediate image, it will be preferable to provide means for correcting for spherical aberration produced by said first section, which, in turn, will provide that the axial locations of its paraxial intermediate image and the intermediate image created by outlying marginal rays will be shifted such that they are closer proximity with respect to one another. It will be beneficial if the longitudinal spherical aberration, SAL, produced by said first section satisfies the condition 0 less than |SAL/L| less than 0.025, where L is the geometric distance between said object plane and the image plane of same, as shall be discussed in greater detail below.
Preferred embodiments of the invention will provide that that surface of that lens of said refractive section that lies closest to said intermediate image will be spherical. However, the surfaces of both lenses facing said intermediate image might also be spherical, which will allow fabricating lenses with high imaging performances and low scatter in their imaging performances without need for imposing extremely stringent tolerances on same, since the figuring accuracies attainable during fabrication are generally better for spherical surfaces than for aspherical surfaces, which also may exhibit transmittance gradients and excessive surface microroughnesses. On the other hand, those surfaces in the vicinity of intermediate images have extremely strongly impacts on corrections for imaging errors, such as distortion, which is why conventional lens designs frequently employ aspherical surfaces near intermediate images. However, in the case of those projection lenses considered here, it will be preferable to employ lenses with high-precision, nearly perfectly accurately figurable, spherical surfaces in the vicinity of said intermediate image.
The previous and other properties can be seen not only in the claims but also in the description and the drawings, wherein the individual characteristics may be used either alone or in sub-combinations as an embodiment of the invention and in other areas and may individually represent advantageous and patentable embodiments.